There has been an increasing interest in the development of laser sources that emit in the mid-infrared (“mid-IR”) spectral region, i.e., at wavelengths between about 2.5 and 8 μm. Such lasers have significant uses for both military and non-military applications. In the military realm, mid-IR lasers can be extremely useful as a countermeasure to jam heat-seeking missiles and prevent them from reaching their targets. In both the military and non-military realm, such mid-IR lasers have found use, for example, in chemical sensing, and so may be very useful in environmental, medical, and national security applications.
On the short-wavelength side of this spectral region, type-I quantum-well antimonide lasers are achieving excellent performance and greater maturity. See, e.g., T. Hosoda et al., “Diode lasers emitting near 3.44 μm in continuous-wave regime at 300K,” Electron. Lett. 46, 1455 (2010). On the long-wavelength side of the mid-IR, intersubband quantum cascade lasers (QCLs) have become the dominant source of laser emissions. See, e.g., Q. Y. Lu et al., “Room-temperature continuous wave operation of distributed feedback quantum cascade lasers with watt-level power output, Appl. Phys. Lett. 97, 231119 (2010).
For the mid-infrared spectral region, the interband cascade laser (ICL) is being developed as a promising semiconductor coherent source.
The first ICLs were developed by Rui Yang in 1994. See U.S. Pat. No. 5,588,015 to Yang. The ICL may be viewed as a hybrid structure which resembles a conventional diode laser in that photons are generated via the radiative recombination of an electron and a hole. However, it also resembles a quantum cascade laser in that multiple stages are stacked as a staircase such that a single injected electron can produce an additional photon at each step of the staircase. See Slivken et al., “Powerful QCLs eye remote sensing,” Compound Semiconductor, pp. 22-23 (2008); see also U.S. Pat. No. 5,457,709 to Capasso et al. Each stage is made up of an active quantum well region, a hole injector region, and an electron injector region. The photon cascade is accomplished by applying a sufficient voltage to lower each successive stage of the cascade by at least one photon energy, and allowing the electron to flow via an injector region into the next stage after it emits a photon. Outside of the active quantum well region and hole injector, current transport in the ICL typically takes place entirely via the movement of electrons, although this is not required. Therefore, two optical cladding regions are generally used at the outsides of the gain medium to confine the lasing mode along the injection axis, and n-type contacts are provided outside the cladding regions to provide for electrical bias and current injection.
ICLs also employ interband active transitions just as conventional semiconductor lasers do. Each interband active transition requires that electrons occupying states in the valence band following the photon emission be reinjected into the conduction band at a boundary with semi-metallic or near-semi-metallic overlap between the conduction and valence bands. Although type-I ICLs are also possible (see U.S. Pat. No. 5,799,026 to Meyer et al., two inventors of which are the inventors of the present invention, and which is incorporated by reference into the present disclosure), most ICLs employ active transitions that are of type-II nature, i.e., the electron and hole wavefunctions peak in adjacent electron (typically InAs) and hole (typically Ga(In)Sb) quantum wells, respectively.
In order to increase the wavefunction overlap, two InAs electron wells often are placed on both sides of the Ga(In)Sb hole well, and create a so-called “W” structure. In addition, barriers (typically Al(In)Sb) having large conduction- and valence-band offsets can surround the “W” structure in order to provide good confinement of both carrier types. See U.S. Pat. No. 5,793,787 to Meyer et al., which shares an inventor in common with the present invention and which is incorporated by reference into the present disclosure. The basic ICL structure was also improved by including more than one hole well to form a hole injector. See U.S. Pat. No. 5,799,026 to Meyer et al., supra. Other improvements are described in R. Q. Yang et al., “High-power interband cascade lasers with quantum efficiency >450%,” Electron. Lett. 35, 1254 (1999); R. Q. Yang, et al., “Mid-Infrared Type-II Interband Cascade Lasers,” IEEE J. Quant. Electron. 38, 559 (2002); and K. Mansour et al., “Mid-infrared interband cascade lasers at thermoelectric cooler temperatures,” Electron. Lett. 42, 1034 (2006).
Further improvements to the ICL structure were made by the inventors of the present invention as described in U.S. Patent Application Publication No. 2010/0097690, the entirety of which is incorporated by reference into the present disclosure, and have improved laser performance in the mid-IR range at temperatures of about 250 K and above.
However, further improved performance in the mid-IR at room temperature remains a goal.